Process for the production of acrylic fibre for fabrics with a low pilling formation and acrylic fibres thus obtained

ABSTRACT

Process for the production of acrylic fibre, for fabrics with a low pilling formation, having a titer within the range of 1.0 to 5.6 dtex, which comprises the wet spinning in a solvent of a copolymer essentially consisting of acrylonitrile, in a quantity within the range of 97 to 99.99% by weight, and a comonomer with a strong acid group of the sulfonic type having the general formula: CH 2 ═C(R)—CH 2 SO 3 —M wherein R represents a hydrogen atom or the CH 3  radical whereas M represents a hydrogen atom or an alkaline metal in a quantity ranging from 0.01 to 3% by weight.

The present invention relates to a process for the production of acrylic fibre for fabrics with a low pilling formation.

More specifically, the present invention relates to a process for the production of acrylic fibre for fabrics with a low pilling formation and the fibre thus obtained.

As is known, acrylic fibre has established a strong position among synthetic and artificial fibres due to its specific characteristics for imitating, and very often exceeding, natural fibres such as wool and cotton.

The processability of acrylic fibre on cotton or wool textile cycles is undoubtedly excellent and much higher than the above-mentioned natural fibres. At the same time, the possibility of easily dyeing acrylic fibres, in bright and light-fast colours for long periods of time, places it at the top of performances with respect to its natural competitors.

Acrylic fibre can also be adopted in numerous end-uses for which it is often appreciated and preferred to natural fibres in fields such as knitwear, hosiery, sportswear, furnishing, awnings, cement reinforcements, etc.

The main field of use of acrylic fibres is undoubtedly knitwear, alone or combined with wool. With use, this type of end-product, however, creates a troublesome problem represented by the appearance of anti-aesthetic pills which increase with use and time.

This phenomenon relating to the appearance of pills is well-known as is also the mechanism whereby they are formed and increase. In the past, the Applicant, for example, studied the formation of pills (Doria G., Trevisan E. “Textile Asia”, 64, 1989) with a kinetic model of the formation, growth and falling of pills up to the proposal of an acrylic fibre, known on the market under the name of Leacril NP, with a high surface corrugation, for delaying the extraction from the yarn of the single fibres, and a low toughness of the fibre for favouring the breakage of the filament which binds the pill to the end-product.

Other producers have proposed acrylic fibres in the “No Pilling” version, according to the above model developed by the Applicant, i.e. fibres with a low toughness and low elongation both longitudinal and also at the knot.

Some of these commercial fibres are obtained from processes which produce fibres “spontaneously” having no or a low pilling formation. Among these, there are processes which use aqueous solvents, wherein, due to the low concentration of polymer in the spinning solution, fibres are obtained with an enormous quantity of latent “spaces” which weaken the fibre. Most of the wet and dry spinning processes, on the other hand, slacken the fibres on-line, rather than in an autoclave with pressurized saturated vapour, thus favouring the spontaneous production of fibres with low knot characteristics.

All the commercial fibres currently present on the market, even though in no pilling and improved versions with respect to traditional acrylic fibres, including Leacril NP, do not completely solve the problem of the appearance of anti-aesthetic pills on the end-products.

All commercial acrylic fibres, in fact, suffer from a “physico-chemical” limit, linked to the composition of the polymer, i.e. the low dimensional stability under wet heat. The cause of this instability of commercial acrylic fibres to wet heat lies in the necessity of “diluting” the CN groups with the introduction into the polymeric chain of a comonomer, for example vinyl acetate or methylacrylate, with a greater molecular encumbrance and ionically neutral, to improve the solubility and processability of the polymer especially with the use of solvents with a lower solvent capacity.

Polyacrylonitrile homopolymers do in fact have an extremely compact structure which is insensitive to hot water and vapour, due to the strong interaction of the dipole—dipole type between the CN groups, making the polymer difficult to dissolve and difficult to transform into fibre. In practice, there are no polyacrylonitrile homopolymer fibres on the market with the exception of Ricem by the Applicant and adopted for technical uses.

In traditional acrylic fibres for textile use, which adopt comonomers of the vinyl acetate, methacrylate type, etc, in percentages within the range of 5-10%, the water molecules succeed in penetrating the fibres and are capable of “solvating” the CN groups acting as a real plasticizer, reducing the fragility of the fibre, constructed with a low molecular orientation (stretch) and low annealing, making the fibre itself more ductile and thus delaying the formation of pills.

The salvation of the acrylic fibre by means of the water molecules takes place both in the dyeing phase of the fibre, in water at a temperature equal to or higher than 100° C., or during use through the humidity of the human body.

Alternative solutions to those described above have been proposed in literature. For example, weakening of the fibre with the introduction of particles of inert materials such as kaolin, silica or TiO₂ (Japanese patent 3,174,012), induction of surface incisions on the filaments (U.S. Pat. No. 3,928,528), pressing of the end-product between two plates with irregular surfaces (U.S. Pat. No. 3,894,318) or reduction of the toughness and knot elongation for a product value lower than 2.0 g/den (Japanese patent application 4057909), maintaining the spinning solution at a polymer concentration ≦20%.

All these solutions proposed have proved to be either ineffective as they produced fibres, from a copolymer, with a low stretch value or with a deterioration in the appearance of the end-product and spinning process. It should be remembered, in particular, that the production of low stretch fibres negatively influences the potentiality of the production system of the fibre.

The Applicant, producer of a polyacrylonitrile homopolymer fibre for use in the filtration of hot fumes from coal power plants, has now found that a particularly modified polyacrylonitrile homopolymer fibre shows an unexpected behaviour to pilling that cannot be compared with any commercial acrylic fibre. This result, described in detail in the enclosed claims, is particularly surprising and unexpected as it is known that a polyacrylonitrile homopolymer fibre is not suitable for a traditional textile use. The Applicant has overcome this drawback by introducing a content of higher dyeable groups with respect to those conventionally obtained by means of a Persulphate-Bisulphite redox catalyst.

The sulfonic and sulphate groups produced by the redox catalyst for a commercial acrylic fibre, containing over 6% of a comonomer such as vinyl acetate or methylacrylate, are normally sufficient for dyeing the fibre also in dark colours, as, due to the plasticising effect of the water and vapour, the dyes easily penetrate the fibre gaining access to all the sulfonic and sulphate groups.

In the case of the polyacrylonitrile homopolymer fibre, it was sufficient to introduce a quantity of additional (to those coming from the redox catalyst) sulfonic groups into the polymerization by the addition of an ethylenically unsaturated sulphonated comonomer, such as sodium methallylsulphonate, to obtain analogous results to those obtained with copolymers based on acrylonitrile for commercial fibres.

With the introduction of the sulphonated comonomer up to a maximum of 3.0% by weight, for example from 0.01 to 3.0% by weight, preferably from 0.1 to 3% by weight, the dimensional stability of the polyacrylonitrile homopolymer fibre under wet heat remains unchanged and the fibre has preserved the “fragility” constructed in the spinning process with exceptional pilling performances.

In particular, the object of the present invention relates to the production of No Pilling acrylic fibre by the wet spinning of a copolymer essentially containing at least 97% by weight of acrylonitrile and a comonomer with a strong acid group of the sulfonic type selected from those having the general formula:

CH₂═C(R)—CH₂SO₃—M

wherein R represents a hydrogen atom or the —CH₃ radical whereas M represents a hydrogen atom or an alkaline metal, such as methylallylsulphonic acid or allylsulphonic acid or their sodium salt, in a quantity up to 3%.

According to the invention, the acrylic copolymer used for preparing the acrylic fibre can also, but not necessarily, contain a neutral comonomer of the vinyl type, for example a vinyl ester of a C₁-C₄ acid, such as vinyl acetate, or an alkyl C₁-C₄ ester of (meth)acrylic acid, such as methylacrylate or methylmethacrylate, in a quantity up to a maximum of 1.5% by weight.

The copolymer used in the process, object of the present invention, is obtained by the polymerization in aqueous suspension with a redox catalytic system of the persulphate-bisulphite type and has a number of dyeable chain-end sites which is such as to ensure an excellent dyeability of the end-fibre in normal textile uses.

The fibre, obtained in a coagulation bath by a dope with a percentage of solids ranging from 17-23% by weight, after wet stretching, drying on hot roles and annealing in vapour, has a minimum breaking tenacity of 26 cN/tex and a minimum breaking elongation of 18%. The guiding values of the loop mechanical characteristics, which prefigure an excellent response to anti-pilling tests of pure and mixed fabrics prepared starting from said fibre (ICI Box Test or Martindale Test), are: loop tenacity ≦4.0 cN/tex, loop elongation ≦1.5% and the product of tenacity x elongation, or loop work ≦6 cN/tex.

The following examples are provided for the non-limiting illustration of the present invention.

EXAMPLE

This example describes the production process by means of wet spinning of the acrylic fibre object of the present invention.

A copolymer containing 98% of acrylonitrile and 2% of sodium methylallylsulphonate is obtained by continuous polymerization in aqueous suspension at pH=2.8-3.0 and a temperature of 50° C. with a potassium persulphate catalyst and sodium bisulphite activator fed in a ratio of 1.30-1.90 expressed as mass flow-rate of activator with respect to mass flow-rate of catalyst. The copolymer, obtained after removal of the non-reacted monomers, filtration and drying of the pellets, has a specific viscosity which, when measured in dimethylacetamide at a temperature of 25° C. in solution 1 g/l, falls within the range of 0.240-0.290. The copolymer has a maximum dissolution peak in dimethylacetamide (measured with a roto-viscometer) at a temperature of 90±2° C., starting from a solvent/polymer slurry at room temperature and a heating rate of 1° C./min.

The polymer thus obtained is dissolved in dimethylacetamide at 18±2% by weight of solid, with respect to the solvent, followed by extrusion of the filtered dope at a temperature of 85° C.

The final titer of the fibre is 2.2 dtex. Dies with a capillary diameter of 52 microns were used, immersed in a coagulation bath at 42° C. and with a concentration of dimethylacetamide solvent in water of 45%.

The tow leaving the coagulation bath is stretched with a total stretch ratio equal to 6× and collapsed on rolls with an internal vapour pressure of 9.0 atm. The tow, after hot crimping, is stabilized with the direct introduction of vapour in the annealer at a pressure of 2.0-3.0 atm and maintains a residual shrinkage of less than 1%.

The breaking tenacity of the fibre thus obtained is equal to 28 cN/tex and the breaking elongation is equal to 23%. The tenacity measured at the loop proved to be equal to 2.2 cN/tex and the loop elongation equal to 1% with the product of tenacity x elongation equal to 2.2 cN/tex (loop work).

The final section of the fibre obtained is round. The fibre in tow form was torn away, transformed into top and then spun to obtain a 100% acrylic yarn and a mixed 50% acrylic and 50% wool yarn (23 microns) in a titer of Nm 2/28-twists 400 z-240 s-.

Two knitted fabrics were subsequently prepared with these yarns, on which the pilling resistance test was effected, after dyeing, according to the standard method BS-5811/86 (ICI Box Test). On a pilling formed evaluation scale ranging from 1 (poor) to 5 (excellent) the marks assigned after 14,400 cycles were 4 and 5 for both the fabric from the yarn containing 100% acrylic fibre described above and also the mixed woollen fabric. 

1. A process for the production of acrylic fibre, for fabrics with a low pilling formation, having a titer within the range of 1.0 to 5.6 dtex, which comprises the wet spinning in a solvent of a copolymer essentially consisting of acrylonitrile, in a quantity within the range of 97 to 99.99% by weight, and a comonomer with a strong acid group of the sulfonic type having the general formula: CH₂═C(R)—CH₂SO₃—M wherein R represents a hydrogen atom or the —CH₃ radical whereas M represents a hydrogen atom or an alkaline metal in a quantity up to 3% by weight.
 2. The process according to claim 1, wherein the solvent for the wet spinning is dimethylacetamide.
 3. The process according to claim 1 or 2, wherein the copolymer comprises a neutral comonomer of the vinyl type, for example a vinyl ester of a C₁-C₄ acid or a C₁-C₄ alkyl ester of (meth)acrylic acid in a quantity up to a maximum of 1.5% by weight.
 4. The process according to any of the previous claims, wherein the unsaturated comonomer of the ionic type with the strong sulphonic acid group is selected from methylallylsulphonic, allylsulphonic acids and the relative sodium and potassium alkaline salts.
 5. The process according to any of the previous claims, wherein the copolymer essentially consisting of acrylonitrile is obtained with a polymerization process in aqueous suspension in the presence of a Persulphate-Bisulphite catalytic redox couple present in a concentration of persulphate salt ranging from 0.4 to 0.6% (as potassium or sodium or ammonium persulphate) and bisulphite salt ranging from 0.6-1.4% (as sodium bisulphite) with respect to the monomers fed.
 6. The process according to any of the previous claims, wherein the copolymer essentially consisting of acrylonitrile is obtained at a temperature ranging from 45° C. to 65° C.
 7. The process according to any of the previous claims, wherein the copolymer essentially consisting of acrylonitrile is obtained in aqueous suspension at a pH ranging from 2.6 to 3.0.
 8. An acrylic fibre with a titer of 1.0 to 5.6 dtex comprising 97-99.9% by weight of acrylonitrile, 0.01-3% by weight of a comonomer with a strong acid group of the sulfonic type selected from those having the general formula: CH₂═C (R)—CH₂SO₃—M wherein R represents a hydrogen atom or the —CH₃ radical whereas M represents a hydrogen atom or an alkaline metal, and 0-1.5% by weight of a neutral comonomer of the vinyl type, for example a vinyl ester of a C₁-C₄ acid, such as vinyl acetate, or a C₁-C₄ alkyl ester of (meth)acrylic acid.
 9. The acrylic fibre according to claim 8, comprising 0-0.4% by weight of titanium oxide with respect to the weight of the polymer.
 10. The acrylic fibre according to claim 8 or 9, having a residual shrinkage in boiling water lower than 1%.
 11. The acrylic fibre according to any of the claims from 8 to 10 having a loop tenacity not higher than 4 cN/tex, a loop elongation not higher than 1.5% and the product of loop tenacity×loop elongation not higher than 6 cN/tex.
 12. A fabric comprising the acrylic fibre according to any of the claims from 8 to 11 and having a minimum Pilling value at the ICI Box Test, according to the standard method BS-5811/86, equal to ⅘, on a scale from 1 to
 5. 13. A fabric comprising the acrylic fibre according to any of the claims from 8 to 11 and from 1 to 50% by weight of a natural fibre, such as wool, having a minimum Pilling value at the ICI Box Test, according to the standard method BS-5811/86, equal to ⅘, on a scale from 1 to
 5. 